I've noticed that Florentine Defense is mentioned in several places in Codex Martialis, but I can't find a description of it anywhere in my book. I'm guessing this is a mistake. How can I get the description of this feat?

"Florentine defense" is an SCAism which refers to using a larger with a smaller weapon. But it's based on two weapon fighting of a type which didn't really exist historically at least not in Europe (not that I'm aware of anyway). The term itself doesn't appear to have any historical basis, I've never come across evidence of people from Florence fighting with say, an axe and a sword or two swords or a mace and a sword or anything like that.

Two weapon fighting was of course used but it's typically with one very short and one much longer weapon, the shorter weapon almost always being a dagger, or a buckler, or some other impromptu device like a cloak or a candlestick.

I need to go through and purge references to it out of the system and replace with the appropriate one, which as I said I think is Main Gauche off of the top of my head.

Yeah, it must be Main Gauche. I'll see if those references to Florentine Defense make sense referring to Main Gauche

Main GaucheTwo weapon fighting style which allows the off-hand weapon to be used defensively. Prerequisites: Weapon focus with both weapons, off-hand weapon must be a Size S or T weapon.Benefit: Add the off-hand weapon’s full Weapon Defense Bonus to Defense against melee attacks for the entire round (max combined +8). Restriction: The off-hand weapon may not be used to attack for the entire round.

On looking closer, I don't think it could be Main Gauche that's meant to replace Florentine Defense. It's only mentioned in a few places, but it's always right along with Main Gauche and seems to be something else. Main Gauche seems to be an offensive feat, where Florentine Defense is defensive.

Here are all the places Florentine Defense is mentioned.

Counterstroke...This feat can be used in conjunction with Nukitsuke, Riposte, or the Main Gauche MF. In this case the off-hand weapon can still be used to defend. Those who have the Florentine Defense feat may use the Florentine Defense with the Counterstroke feat.

Two Weapon Disarm (FMA feats)May use an offhand weapon in the second hand in conjunction with your primary weapon to disarm an opponent without drawing an attack of opportunity.Prerequisites: Two-weapon fighting feat. Florentine Defense martial feat. Restrictions: requires at least two dice remaining in Martial Pool, you must be at Melee Range to attempt this.Benefits: If you have two weapons and your opponent only has one, you may attempt a Two-weapon disarm. Attack with your first weapon, if your opponent successfully makes an Active Defense roll, you may attempt to disarm with your offhand weapon without drawing any Attack of Opportunity.

I just found Niten Ichi. Is that the one that's supposed to replace Florentine Defense? The descriptions sound similar. If it is, it seems much better than Main Gauche. It requires one less feat (though you do need a higher BAB) and gives you the same benefit while allowing you to attack with both weapons instead of just one.

Niten Ichi (‘Two Swords as One’)The Nitchen Ichi is a two-weapon fighting style which allows both weapons to be used defensively, together. This is named after Musashis system for fighting with a Wakizashe and a Katana simultaneously.Prerequisites: Combat Expertise, BAB of 6 or better. One of the two weapons must be size S or smaller. Benefit: You may combine the Defense Bonus of both weapons being used for a combined maximum of +8 to Defense while still attacking with both weapons if you wish to do so (though you gain no extra attacks or MP). This stacks with Combat Expertise. The combined defensive bonus remains in effect even at grapple range.